Dear NT:

You wrote:
> Is this sentence correct:
>
> He did the magic trick SO SLOWLY THAT we SHOULD see how he did it.
>
> (Indicating purpose and manner.)
>
It is OK, but a native English speaker would probably say “we should be able to see” or “we could see.” Also because the main clause is in the past tense, you would normally say “should have been able to see” or “could have seen” to keep the tenses aligned.